ISA is an Individual Savings Account..
The interest gained from an ISA is tax-free. There are two types of ISA: mini-ISA and maxi-ISA.
An ISA can be made up of an investment in cash, or longer term investments like stocks and shares or insurance. In each year you can either invest in one Maxi ISA, which can include all of these types of investments, or you can have two Mini ISAs one for cash and one for stocks and shares, which can both include insurance.
You cannot invest in both a Mini ISA and a Maxi ISA in the same tax year. For further information go to the HM Revenue & Customs website, see link below.
HMRC website - factsheet on ISA's (External Link)